How to adjust object geometry to enable bevel of two edges leading to a pointed corner of a triangle?
Many modifiers and other tools in Blender are designed with quads in mind. This is why creating n-gon free and triangle free topology has become a serious domain of craftsmanship on its own. Some people hate that about 3D modelling, some love it. It reminds me of figuring mathematical problems or solving puzzles. There are many guides on creating the best topology, here's an example: Topology Guides
Looking at your mesh, it requires cleanup:

But also it requires dealing with the triangle, as your problem still happens after the cleanup:

Originally I thought you mean something else, as the stretching above is to be expected when the topology has triangles or n-gons.
Considering, how simple the shape is, it's best to just start over instead of fixing it. What I learned to do, is to to think of triangles as unnatural, as very often a triangle will produce a perfectly sharp corner, which just doesn't happen in the real world. Therefore I try to replace triangles with quads having one very short edge. In your particular case, you could start with an octagon:

Decreasing every second of its edges to almost 0, Then selecting a pair of corners on the opposite sides, pressing J to join them (creating an edge and also dividing the face this edge goes through into 2 faces). Then the same for the other pair of vertices, then creating a perpendicular loop cut with CTRL + R, then two more loop cuts, this time parallel, then selecting 2 vertices and moving them up, and finally selecting two inner vertices and scaling them so they move underneath the other two. This is still not perfect technique, but I tried to make the technique easier at the cost of worse result.
The result should be a &quot;triangle&quot; that actually is a quad upon closer inspection:

You can now bevel the edge without problems:

If you now create triangles by moving those vertices together:  , the bevel will still work - but only until you decide to remove doubles (merge vertices by distance) - then creating new bevels will malfunction again.